but what's this talk about 5k 27" monitors (or something like that) needed for readability on Macs and that 4k 27" wouldn't work well?
TLDNR: If you want the
optimum display quality for your Mac (see below), pay the price and get a 5k 27" or a 6k 32" Pro XDR. If you're going to be spending 8 hours a day staring at this thing, that's not necessarily stupid. If, however, you want to save $1000 or so, or want a dual-display setup for the price of a single 220ppi display, or fancy a different size/format of screen, buy a 4k display and enjoy a
perfectly good display for most purposes and don't start seeing problems because YouTube told you to.
Today's Mac OS UI is designed to look its best at 220 pixels-per-inch. That's what you get with a 5k 27" (5129x2880) display like the Studio Display and the Samsung being discussed here. The Pro XDR display - 32" at
6k also works out to 220 ppi, the 24" iMac at about 210 ppi, the various MacBooks vary between about 220-250 ppi so its
almost a constant 220ppi across all Apple displays. People sometimes refer to this as a "2x" or "pixel doubled" mode because everything is using 2x as many linear pixels as an old-school 110ppi display (like the old pre-5k iMac) - you
can (by jumping through a few hoops) switch to "1x" mode which, on a 5k display, will leave all of those icons, fonts etc. unusably small.
The practical upshot of this is that icons, menu bars, system fonts, dialogue boxes, scroll bars etc. all appear to be about the same
physical size (i.e. if you hold a ruler up to the screen) across all Apple displays, and any bitmap assets (icons etc.) included in apps will have been created at that resolution.
If you, instead, use a 27" 4k display, the resolution is more like 160ppi and, in the usual "2x" mode, all those icons, system fonts etc. will consequently appear a bit large and occupy more screen space. MacOS gives you three options:
First, you can switch to 1x mode with tiny system fonts, icons etc. which is
just about usable with young eyeballs on a 27" 4k - but if you go for a 30" or larger 4k display it starts to become a viable option (although, with that, comes larger and more visible pixels).
Second: you can use a "fractionally scaled" mode - the most common one being the misleadingly named "looks like 2560x1440" which essentially renders everything to an internal 5k virtual display and then downsamples the result to 4k - and no, it really doesn't "look like 2560x1440" - unless you've got better than 20/20 vision or climb up on the desk and do direct comparisons with a 5k it looks pretty close to 5k. There
are some downsides - the downsampling puts some extra load on the GPU (maybe a problem with the old Intel iGPUs but Apple Silicon shouldn't break a sweat) and there is some degradation of single-pixel features (but 160ppi at 21" viewing distance is 'retina' so those are going to be hard to see).
Third:
come on folks: 2x mode is perfectly usable on a 27" 4k. Its just that the system icons and dialogues are a bit large - but hide the dock, auto-hide the menu bar (or work in full screen mode) and almost
every app lets you zoom the actual content exactly to taste, so if you want lots of spreadsheet rows or lines of code its a non-issue. I know people who run in this mode on a 27" display because it is easier on the eyes than "looks like an iMac" mode... and you only
need to use this mode if you're doing some specific job for which fractional scaling is an issue... and if you're after screen estate
you can get 2-3 display setup for the price of a single 5k display.
Unfortunately, there are a couple of articles out there which hype up the problems with fractional scaling to absurd attempts (...e.g. showing greatly enlarged screenshots of pathological cases like 1-pixel grids and completely ignoring the possibility of taking 5 seconds to change screen mode when dealing with such). Yes, 4k is a compromise - but its a perfectly sensible compromise for many.
NB: The big problem is that Windows (and many Linux UIs) don't have the Mac's fixation on 110/220 ppi
or the Mac's history with 1440p 27" displays - instead, they basically let you set the system's true ppi resolution and everything gets scaled accordingly - this does have its disadvantages though (it relies on applications being well behaved - and pre-rendered bitmap assets will still get rescaled). Also, the pre-retina "standard" for PCs, which may have influences software design, was more like 1080p than 1440p so doubling that to 4k probably
is the sweet spot for Windows. Plus, Apple pioneered 5k with the iMac which used a custom, internal, high-speed video interface. The external 5k displays for PC that appeared around that time used a kludgey two-cable DisplayPort connection. Anyway, the PC world never really took to 5k, leaving it as a niche product for Mac users, hence the expense.
